Crossbar switch



Nov. 28, i 944.

V c. GILLINGS ET AL CROSSBAR SWITCH Filed Dec. 26, 1942 10 Sheets-Sheet 2 LSU 4 INVENTQRS CHARLES GILLINGS LAWRENCE JOHN MURRAY ATTORNEY Nov. 28, 1944. c. GlLLlNGS ET AL 1 & 2,363,955

CROSSBAR SWITCH I Filed Dec. 26, 1942 10 Sheets-Sheet s INVENTORS CHARLES GlLLlNGS LAWRENCE JOHN MURRAY ATTORNEY NOV. 28, 1944. c, 5 N 3s ETAL 2,363,955

' GROSSBAR SWITCH 1 Filed Dec. 26, 1942 10 s eets-sheet 5 CHARLES GILLI N6 8 LAWRENCE JOHN MURRAY 1 ATTORNLY Nov. 28, .1944. c. 'GILLINGS ETAL 253633 A I CROSSBAR SWITCH Filgd Dec. 26,1942 1o Sheets-Sheet 6 mvErwoRs "CHARLES GILLINGS LAWRENCE JOHN muanm ATTORNFY Nov. 28, 1.944.

0. GILLINGS ET AL CROSSBAR SW'ITCH Filed Dec. 26, 1942 10 she etsfshe et 8.

CHARLES GJLLINGS LAWRENCE JOHN MURRAY BY .1 ATTORNEY Nbv. 28, 1 944. c. GILLINGS ETAL- 2,363,955

. GROSSBAR SWITCH Filed Dec. 26; 1942 U 10 Sheets-Sheet 9 SCE 1 m VENTORS CHARLES GILLINGS LAWRENCE JOHN. MURRAY ATTORNEY Nov. 28,- 1944. c. GILLINGS ETAL 2,363,955

CROSSBAR SWITCH I Filed Deb. 26, 1942 10 Sheets She et 1L0 IINVENVTORS CHARLES saunas LAWRENCE JOHN MURRAY ATTORNEY" Patented Nov. 28; 1944' oaosseAa SWITCH Charles Gillings and Lawrence John Murray, Liverpool, England, assignors, by mesne assignments, to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application December 26, 1942, Serial No. 470,158 In Great Britain February 27, 1942 13 Claims. (Cl. 179-22) The present invention relates to novel methods of employing crossbar switches in telephone or like systems and has for its object arrangements by which the number of crossbar magnets and contact sets required for a given size of exchange or'switchboard may be kept at a low value. The kind of crossbar switch to which the invention relates is one in which a preparatory operation corresponding to a row of contact sets is first made and subsequently a holding operation common to a column of contact sets is made whereupon a particular contact set corresponding to that which belongs both to' the row and to the column, for which preparatory and holding operations have been made respectively, is operated. When this type of crossbar switch is used in automatic telephone circuits it is usual to employ two of such switches for a group'selecting stage or for a combined preselecting and final selector stage and it is one of the features ofv the present invention to extend the application of a single crossbar switch to such conditions by arranging that instead of one preparatory operation only being effected two are effected whereby a connection can be established between operationis determined independently of both distinctive preparatory operations. For instance, one preparatory operation may depend upon the line of a subscriber taking part in the connecing crossbar switches in which a group of subscribers lines and groups of incoming and out;

can be made between any of a number of lines incoming to the switch and any line of a number bf groups of linesoutgoing from the switch whereby a complete connection can be set up. employing a single crossbar switch for the preselecting and each numerical stage.

The invention willbe better understood from the following description of one method of carrying it into effect reference being had to the accompanying drawings comprising Figs. 1-10.

Of these figures Fig. 1 shows the essential trunking arrangements of a crossbar private automatic exchange board, hereafter termed P. A. X board, which has a capacity oi up to 100 lines. Fig. 2 shows the cross connection arrangements between the various 20 line groups and the five link switching units .on the board. Figs. 3-10 when arranged in themanner shown in Fig. '6 show the circuits of the board.

Referring now to the trunking diagram Fig. l, the IOU-line board comprisesa number of 20-line groups up to five as a maximum, and bearing the tion and't'h'e other preparatory operation may depend upon whether the said subsc'ribers line is calling or being called, while the holding operation is dependent upon which of a number of lines in an outgoing'or incoming group the connection is to be made with; this, applies to a single crossbar switch employed as a combined preselector and final selector switch. Oragain one preparatory operation may depend upon the line over which connection is made to said switch which line of the selected group connection is to be made with. Such a crossbar switch can be used as a group selector.

A further feature of the invention consists oi an automatic telephone or like system employtwp digit numbers 11-20, 31-40 up to 91-00. For each set of 10 lines a small crossbar unit or so-called subscriber switching unit SSU is provided, these units being grouped together in pairs v The two units SSUi and SSUii serving subscribers 11-10 and 21-30 respectively are shown with special reference to outgoing calls, while g the units SSUEi and SSUflll serving subscribers 91-90 and 01-00 are shown with special reference ringing relays, etc, are associated with each link switching unit; through each such unit up to four connections can extend at any one time, although incommon with other crossbar switching units it will be appreciated that such calls are not set up simultaneously. For each 20-line group comprising a pair of subscriber units such as SSUI and SS UZ there areprovided four outgoing links and also four incoming links. The outgoing links from all subscriber units are distributed over the five link switching units LSUl-LSU5 in the manner shown in Fig. 2 while the incoming links are available from the outgoing side of each of the live link switching units on the board. The arrangements are such that 7 ject to a maximum when four outgoing and four.

incoming connections are involved. lhese arrangements enable the board to provide a high grade of service at reasonable cost, but it will be understood that the invention is not limited in scope to the precise number of links stated as this maybe altered to suit the 'traffic requirements involved.

Reviewing now the essential construction of the various crossbar subscriber units SSUI- SSUlll, these each comprise ten horizontal operate bars and the associated bar-controlling relays ISP-IOSP which connect with the ten subscribers' lines concerned and also four vertical hold bars and the associated relays ASI-L-DSH internal links between the calling connecting link circuits and the links taken in use leading to the called 20-line groups.

In order to prevent the various links involved in a, connection from being confused with one another. it is proposed that the links which terminate at one end on the outgoing side of the sub-,

scriber switching units and at the other end on the incoming side of the link switching units,

and which will each include a connecting, link circuit, shall be termed out links; and that the other links which terminate at one end on the outgoing side of the link switching units and which control the extension of the four possible ated with the particular free outgoing link select-,

ed, the vertical hold" bar providing in-efiect an internal link within the calling subscriber.

unit. The connection of an incoming link to a subscriber is effected in similar manner. except that relay IP is operated this time in addition to the relevant SP relay, after which the 'vertical hold relay associated with the particular link calling'is operated to provide the necessary-internal link within the called subscriber unit.

As regards the link switching units, these each comprise four horizontal "operate? bars and the associated relays lCP-ICP with each of which-a' connecting link circuit is associated. Five additional horizontal operate bars and associated relays lTP-5TP are provided for the purpose of selecting the particular set of links incoming to the required 20-line subscriber group, the particular one of these relays to be operated being determined inaccordance with the tens digit of the particular number dialled. For instance, for a tens digit 1 or 2, relay lTP- will be operated to prepare for connection to any one of the four links connecting with the first 20-line group on the board, that is to saythe subscriber units' SSUI and SSU2, while if a tens digit 9 or 0 is dialled then relay 5T1? willbe operated to prepare for connection to any one of the four links extending to the 20-line group comprising subscriber units SSU9 and SSUI ll. Each link switching unit may therefore be said to constitute a five-level four outlet per level, group selector switch which atthe other end on the incoming side of the subscriber switching units, shall be termed in links. The vertical bar links within the subscriber. switching units and within the link switching units may be generally described as internal'vertical bar links of the switching unit in question.

Referring now to the circuits of Figs. 3-10, for

be understood are shown in Fig. 3. From reference to the line circuit of subscriber No. 11 it will be noted that as previously mentioned, the line of this subscriber will connect with the horizontal bar relay ISP or the crossbar subscriber switching unit SSUI, which. relay ISP serves as the subscriber's line relay so that each subscriber will only'require a single individual cut-off relay- I The primary control equipment PCEI is provided with a rotary switch such as IK in addition.

of the self-interrupted drive type which is adapted to select a fre one of the four possible out links to which the subscriber group'in which the calling party is located has access and thereafter to cause the crossbar subscriber switching unit in question to switch through to the selected link. Fig. 4 shows a connecting link circuit such as AL of Figs-1 and 2'which is assumed to be associated with the first of the four switching unit LSUI and its associated controlling relays. .Common to each of the'flve link switching units on the board are the secondary control equipments SCEl-SCES shown in Fig. 2, circuits of SCEI being shown in Figs. 8,9 and 10 and being accessible over common leads from each'of the connecting link circuits AL-DL-associated with link switching unit LSUI.

0n the taking into use'of the circuit AL: this is connected over common leads with equipment SCEI common to the connecting link circuits AL-DL of the unit LSUI while the unit LSUI horizontal bar associated with the calling link is actuated to prepare for connection'to any one of the four available in links to the required 20-line group which may be selected.

The dialling of the tens digit of the called number into equipment SCEl enables the required group of in links to be determined and at the end of this digit, connection to the selected. group is prepared for by the operation of the relevant TP relay which actuates its horizontal bar. At this stage therefore the horizontal bar associated with the calling out, link and the horizontal bar associated with the selected group of "in links areboth actuated but no connection can be effected therebetween until the vertical hold bar representing the internal link within the unit is operated.

The dialling of the units 'digit enables the required subscriber's line to be ascertained and tested from equipment SCE] and, if found to be free, to be connected with via the unit LSUI and the subscriber unit in which the line is located. In order to prevent more than one connection being in process of being set up through a sub-.

scriber unit at a time and so to prevent multiple connection to an outlet, the above-mentioned switching-through operation is controlled by a a 'so-called final switching control equipment FSC,.

common to the board, which is accessible to the SCE equipment in question when both tens and units digits-have been dialled. The equipment FSC on being taken into use initiates testing for a free one of the four available in links to the called 20-line group as determined by the particular tens digit dialled, the testing arrangements being so devised that an in link will not a test free unless the corresponding internal link within the unit LSUl is also tree, this being necessary since it is possible that the internal link might already be in use on a connection to some other 20-line group. If a free "in link together with a free corresponding internal link in the unit is encountered, then the relevant vertical bar will be actuated to effect the necessary internal connection between the calling out? link and the'seizecl in link. At the same time a 'signal is sent forward via equipment SCEII to operate the incoming relay m of the calledsuh- .scriber unit of the pair sharing the four in links and on which the called party is located, whereupon the associated horizontal bar is actusteel. The equipment SCEl is'also adapted at this stage to test thecalled line and if it is found to be free, then the associated line relay is operated and actuates its horizontal bar. When the testing relay in the equipment SCEl operates to indicate that the called subscriber is free, the called subscriber unit vertical bar internal link associated with the seized in link is actuated to connect the in link through to the called subscriber at the subscriber unit, and since the call ng out link will have already been connected through to the other end of the in link at the link switching unit, the connection beeach relay is shown underneath its designation as for instance relay (Fig. 3), and these contact units are designated individually in small letters which in this instance will be lki, Hat and i A slow-to-rele'ase relay is shown with a hatched portion at one end of the rectangle indicative thereof, while a slowto-operate, relay with a copper slug at the arma ture end or its core is shown with a cross at oneend. The contact units individual to the cross- 3 bar switch vertical and-horizontal relays are indicated in the same manner, while the set of crossbar switch contacts proper which are closed at the intersection of any given pair of vertical 7 and horizontal bars are designated IP SP t e C. i 7' according to the particular crossbar relays involved.

The various rotary switches such as thatin the common equipment of Fig. 7 which is designated HS are provided with driving magnets such as HM each having a set of interrupter contacts such as hm. The various sets of wipers and banks thereon are designatedHSl and H52 etc.

Considering now the circuit operations, it will be assumed for the purpose of the description that subscriber No. 11 originates the first call on the P. A. X board in which case the loop placed via his telephone across the left-hand top set of incoming negative and positive conductors (Fig; 3) operates the combined line relay and I crossbar magnet lSP of the subscriber unit SSUl over the following circuit: battery, resistance YA, chain-connected contacts lspt-Itspll of the relays lSP-lllSP on the unit SSUl, contacts tlcrl and I892, upper winding of relay lSP, contacts Hci, negative line, subscribers instrument, positive line, contacts IE2, common lead 9 on to which the positive lines of all ten of the subscribers 11-10 are connected, contacts of the vertical hold relays ASH-DSH in the two subscriber incoming test lead T. It will be noticed that five relays lS-5S are provided in association respectively with the sets often SP relays of the subscriber units SSUi-SSUd, and in the present instance relay lSP of subscriber unit SSUl in operating completes an operating circuit for relay is over the following circuit: earth, resting contacts z'cst; common lead ll, resting contacts fist- 5st, resting contacts 5st and fist, winding of relay is and operated contacts ispt to battery via resistance YA. Relay is in operating in a manner to be described serves to ensure that the hunting switch HS in the common unit of Fig. 7 shall only test'oh to the group of four out links to which the subscriber units SSUtl and SSU2 have access, while at contact Isl it first completes a local locking circuit for itself and at contacts ls'll and lst it disconnects any operating circuit for the relays 2S-5S of the non-calling subscriber units SSUt-SSUb in case subscribers on these units endeavour to call at this time, it being necessary to ensure that only one call shall be in process of being set up at any one time on ,a group of crossbar units served by one common equipment. At contacts is? it also complates acircuitto relay SA which is common to all fifty subscribers associated with subscriber units SSUl -SSUE and serves to initiate the operation of the common equipment of Fig. 7. Relay .SA, which, although shown in Fig. 3, really forms part of the equipment of Fig. 7. in

with which the calling party is permitted to be associated, the selection being determined by the particular S relay operated. With relay IS opactuatcs its associated horizontal bar on subscriber unit SSUI to prepare for connection of the "out link to link circuit'AL and at its contacts opI (Fig. 7) extends earth over wiper HSI in position I to bring up the subscriber unit SSUI vertical hold relay ASH associated with the first out link. Relay ASH in operating actuates its associated vertical bar so that the sets of crossbar contacts at the intersection of the vertical and two horizontal bars in question will be operated. These are shown as erated, relay TK is only connected up to wiper I HS3 when the switch HS is in any one of the positions I-4 where it gains access to the first group of "out links, while if relay-2S were operated the same remarks would still apply, since subscriber unit SSU2 also has access to this same group of links, excepting that in this instance access to the group is had over HS3 bank contacts. 5-43. If relay 3S were operated then access in Fig. 3, and are numbered I-3 and I-4 respectively, the contacts I-3 in closing extending the calling line circuit through to the connecting link circuit taken into use. Relay ASH in operating also at contacts ash5 opens the intial operating circuit for relay TK (Fig. 7) but this relay is held via contacts ashB (SSUI) (which would be possible to the second group of links 1 over HS3 bank contacts 9-I2. In the present instance with relay IS operated and with the HS switch wipers in position I, a testing circuit is completed from earth via the winding of relay 'I'K, contacts saZ, wiper HSZ on bank contact I, contacts 41.9725 and Isl and wiper HS3 on bank contact I which extends over the first of the four out -links accessible to subscriber N0. 11

to connecting link circuit AL shown in Fig. 4. It is assumed in thepresent instance that this. link circuit is disengaged so that the corresponding outf link will also be disengaged and it will be marked free by a resistance battery potential extended thereto from the secondary control equipment SCEI of Figs. 8-10, which, as previously described, is common to the link switching unit LSUI with which four connecting link circuits including link circuit AL are terminated. The idle marking battery potential extends from battery via resistance YH (Fig. 8), contacts nI and MI, common lead I6, and contacts $112 and sell (Fig. 4) onto the test lead P of the out link in question. Relay TK (Fig. '7) quickly operates in series therewith and at its contacts tkl opens the energising circuit of magnet HM before it has time to operate so that the HS switch wipers will remainin position I.

If link circuit AL proves busy, then an earth potential will be fed back over .the P lead to prevent relay TK from operating and the HS switch I where the testingoperation will be repeated and make before contacts ash5 open) and tkrB to battery via resistance YG. The operating earth for relay ASH extends over contacts ashl to the P lead to guard the seized link and also extends over sets of crossbar contacts 0P sP 3 ASH Asa to bring up the calling partys cut-off relay IK. This relay in operating at contacts IkI and Ik! removes the line relay ISP from across the speak- 3 and ing leads and this relay releases while at I103 it maintains the guard earth on the incoming test lead T so as to maintain the busy marking of the calling line. Relay ISP in releasing opens the holding circuit of relay IS at contacts Ispl horizontal preparerelay ICP (Fig. 4) associated with link circuit AL and located .on link switching unit LSUI. At contacts b3 dial tone is connected up to the centre winding of relay 6. over the following circuit: common lead 25 so on- If all the connecting link circuits accessible to subscriber No. 11 prove busy, then the HS switch will continue rotating and in each revolution will test the four outlets accessible to this subscriber until. one becomes available. Al ternatively, it might be arranged that switch HS after testing the four releyant outlets and finding them busy would proceed to an unused position and thereupon bring about the operation of a relay to return a busy tone to the subscriber. who would have 'to bang up his receiver and call again later. e I

If link circuit AL'is free, relay TK operates and at its contacts tkI brings up relay TKR which at'its contacts tkrI-tkr5 (Fig. 3) opens the initial energising circuit of all SP relays, and at contacts tkr'l completes a circuit over operated contacts Is5 to operate the subscriber unit SSU I outgoing prepare relay OP. This relay in operating (F18. 8) on to which an earthed dial tone source is extended, contacts n3, common lead 2I,-and contacts sdi, b3, scl and 12 to the windingot relay A, dial ton thereupon extending by induction through the windings of relay A on to the speaking conductors. At contacts M a guarding and holding earthpotential is extended on to the link test lead P, and this earth extends'back to hold the cut-off relay IX of the calling line circuit and relay ASH before relay IS has had time to release. On release of relay IS, theholding circuit o1';relay'SA is opened at contacts I32, while at contacts Is5 (Fig. 7) relay OP releases the associated horizontal bar. The pin or its equivalent which is engaged by the vertical bar remains held without appreciably restraining the restoration of the horizontal bar. The same remarks also apply in the case of the horizontal bar assorelay IK. Relay SA in releasing opens the holding circuit of relays TKR and TK (Fig. 7) and with the release of relay 'I'KR. the board is now in readiness to deal with any further calls from subscribers in the 0,-line group the equipment PCEL'. -31 v The unit LSUI horizontal prepare relay .ICP

in operating also'at its contacts M213, which are shown for .convenience in Fig. 4; completes an operating'circuitv over common lead 19 for relay BA (Fig. 8). in the equipment SCEI, while other contacts 'lcpl, lcp2. [c104, further connect they link circuit AL through to the equipment SCEI'.

Relay BA in operating at contacts bal (Fig. 8) disconnects the idle resistance marking battery from lead I6 extending through to the link P lead and at contacts bad extends earth on to the j start lead ST of a delay period timing set common to the board, while at MS it completes an energising circuit for relay DC- over both its windings in series with the tens switch driving magnet DM (non-operative in this condition) Relay DC in operating at its contacts dcl connects the impulsinglead from th connecting link circuit through via the low resistance winding of relay DC to magnet DM in readiness for the dialling of the first digit of the wanted number.

The calling subscriber No. 11 on hearing the V dial tone should now proceed to dial the two required digits of the numbenbut in case he should neglect to dial and so hold common equipment SCEI unnecessarily, arrangements are made to release this equipment and to return busy tone to the calling subscriber if dialling does not take place within a predetermined period after seizure. This is effectedin the following manner. On the extension of earth to the delay set start lead ST,

" the delay set is set into operation andsoon afterwards applies an earth puls on to common lead S, extending therefrom; Relay TA thereupon operates on its left-hand winding and locks to earth on its right-hand winding, while at contacts ta2 it connects its middle winding and the lower winding of relay TZ up to the Z lead. After a predetermined period an earth pulse isapplied from the delay set to the Z lead, whereupon relay TZ comes catered for by lay B being short-circuited during this time, while the lower winding of relay DC is similarly short-circuited in series with resistance YM. On

the re-operation of relay A, an energising circuit is re-com'pleted for relay B and relay, DC now holds operated an both its windings in series with magnet DM; which will not hold in this condition, and thereupon releases to advance its wipers from contacts I to contacts 2. An immediate operating circuit is now completed for relay N over wiper and bank D8! and contacts M6 to battery via resistance YK and this relay at its contacts nl further disconnects the idle marking battery circuit and at 112 opens the holding circuit for relay DC. and'at the same contacts short-circuits its lower. winding so thatii there should be no second impulse of the train, relay DC will release after a slow release period governed by the shortcircuiting' of its',winding and will initiate the changeover of the impulsing path from the tens switch magnet DM to the unit switch magnet UM (Fig. 9). In like manner, as described, relay B holds operated during each succeeding break impulse and it will be mentioned here that the presup on its lower winding in series with relay TA which holds on its middle winding, the initial operating and holding circuits for relay TA being disconnected at contacts 2223 and $22. Relay '12 in i operating at tel removes the short-circuit from its upper winding and thereupon looks over this winding in series with resistance YJ, while at tel earth is extended over common lead H to bring up relay SD in the link circuit, At tel earth is extended over commen lead It to operate relay SE in common lead iii to the centre winding of relay A from whence it returns to the calling party who will have to hang up and call again when he decides to set up.a connection;

If the calling party commences to dial within the predetermined period after receipt of dial tone, which-period is long enough to allow for a reasonable delay in dialling, then on the first release of relay A in the link circuit an energising circuit is completed for magnet DM (Fig. ,8) as follows: earth, contacts bl, al and lcpl, lead 20, contacts dc l, low resistance upper winding of relay DC, magnet DM to battery, the winding of reence of the metal rectifier MBA in its energising circuit serves to prevent the slowing up of the operation of magnet DM which would otherwise result by the direct connection of a comparatively low value non-inductive resistance in parallel with its winding; neither the magnet DM nor relay DC moreover will hold operated in series with relay B.

At the end of the first train of impulses, relay A remains operated and holds relay 3 while relay DC commences to release slowly, relay N remaining held over wiper and bank DSI. On release of relay DC the impulsing circuit is changed over at contacts do! on to magnet UM of units digit "switch US, relay UC operating on closure of contacts (102 in similar manner as for relay D0,.and at contacts ucl completing the impulsing circuit. In orderto avoid the use of ten unit switches for dealing with the units digit of any of the possible ten ill-line groups of subscribers on the the link circuit and at tzithe operating circuit for from equipment SCEI and at contacts sdl and sefi connect a number unobtainable tone source via exchange, arrangements are made whereby if an odd tens digit is dialled then the units switch will operate over the first half of its bank,'while if an even tens digit is dialled the units switch will operate over the second half of its bank. By thismeans only, five units switches are required for the lines. This is'eflected in the following manner. If in response to an even tens digit the DS switch has been set to any of-the positions 3, 5, I, 9 or H or l5, l1, 19, 2| or 23 according as to whether it is operating from its first home position I or its second home position l3, then on the release of relay DC during the interdigital pause, earth will be extended via the low: resistance winding of relay N (Fig. 8), contacts ball and (103 (Fig. 9), bank and wiper DSZ,

bank and wiper U52 in normal position, units switch magnet interrupter contact um, and magnet UM, to battery. Magnet UM in operating in a self -interrupted circuit thereupon advances the units switch wipers. to position 2 after which a. ,self interrupted driving circuit is completed via contacts bal and bank and wiper'USZ to advance the wipers to position l3 where the driving circuit is opened at wiper and bank USZ. If an odd tens digit is dialled, it will be seen that no driving circuit can be completed for the magnet UM and the US switch wipers will remain on position I in readiness for the units digit.

It will be assumed that connection is to be made to subscriber! and in response to the dialling of digit the units switch will have been positioned during the interdigital pause to position I3. When the units digit 1 is dialled, relay A in releasing energizes magnet UM and when relay Are-operates at the end of the impulse, magnet UM releases to advance the US switch wipers to position 2. As regards relay UC, during the release of relay A the lower winding of this relay is short-circuited, while on the' reoperation of relay A, relay .UC maintains itself operated on both its windings in series with ma net UM which will not hold in this condition. Wiper USI is of the bridging type so that this holding circuit will be maintained until the wiper engages with contact 2 when the lower winding of relay UC will be short-circuited. If only one impulse is involved as in the present instance, relay UC will now commence to release slowly due to its short-circuited winding and on its release the impulsing circuit will be opened at contact ucI. Since relay N.wlll still be held operated over wiper and bank DSI, an energizing circuit will now be completed for rela'y' SGA (Fig. 9) over the following circuit; earth, contacts 115, M9, ,fzrZ, dci, uc2 and n5, winding of relay SGA to sary switching through from the calling (301mm.

ing link via the link switching unit to the free in link can be effected. A further point is that for each in link in the selected four link group there is a corresponding internal link in' each of-the two terminating 10-line subscriber units sharing the group, and an in link can therefore only be marked free if the corresponding internal link in the called subscriber unit is not in use either on an incoming or an outgoing call and if the corresponding internal link inthe other subscriber unit of the pair forming the 20- I ,line group'is not in use on an incoming connecbattery. Relay SGA in operating brings into operation the final switching control equipment FSC (Fig. 8) common to the board, which serves to control the testing of the wanted line and at the same time to ensure that only one connecting link circuit on the board can perform a called line testing operation at any one time, so' as to prevent the possibility of multiple connections to one outlet. Relay SGA in operating at its contacts sga3 (Fig. 8) extends earth over common lead'24 to bring up relay FS whichin operating at contacts is! completes a self-interrupted driving circuit for the magnet SCM of switch SCS in the common equipment and this switch thereupon rotates its wipers in search of a marking indicative of the calling equipment SCEI, which tion, it being appreciated that if the latter internal link is already engaged on an outgoing connection then the in link need not be marked busy. This point is taken care of by arranging to provide odd and even test leads for each in link, the test lead to be utilised being determined in accordance with the particular tens digit dialled. When testing over an odd-test lead the busy or idle condition of the internal link within the odd subscriber unit of the pair inquestion as far as incoming or outgoing calls are concerned together with the busy or idle condition of the internal link in the even subscriber unit of the pair as far as incoming calls are con gether with the busy or idle condition of the inmarking comprises battery potential via resisttemal link within the odd subscriber unit of the pair as far as incoming calls are concerned is uated on the even subscriber unit SSUIO and the testing for the busy or idle condition of the four in. links extending to this subscriber unit will be performed, over the 01-00 even group of test' leads on to which access can be had over the T35 wiper and bank. The TS switch is arranged -test lead of 11-10 group of test leads to subwill operate immediately and at its contact i102 will cut the self-interrupted driving circuit for magnet SCM before the magnet has time to en- 1 ergise. At contacts ,flcl earth is extended over wiper and bank SCS2 to bring up relay FZ in the equipment SCEI and this relay at contacts 123 twenty subscribers in which the called subscriber is situated, the group being determined in accordance with the setting of the tens switch DS. The in link testing arrangements are moreover such that the wipers of the testin witch TS will only halt on a link having a correspondin vertical bar internal link within the link switch unit LSUI also disengaged so that the necesscriber unit SSUI of Fig. 3 is shown and is designated X, it being appreciated that other test.

leads will terminate at the relevant subscriber units in like manner.

Considering now the circuit between the testing relay KR (Fig.- 10) and the X test leads, it

-is occupying positions I 8-2l, while furthermore wiper andbank DS5 determines that wiper and bank TSS and not wiper and bank TSB shall be utilised sothataccess will be had to the even group of test leads III-00. It will be furthermore noted that in the circuit to each of the four branch test leads to TS3 there is interposed a normally made contact of each of the four vertical bar relays ICH-ICH so that if any of these links should be engaged then the circuit to the vertical link in unit'LSUl.

2,363,955 .associated test lead will be broken and the corand wiper D54, wiper and bank DS5, wiper and bank TS and the first even test lead of the group which will be marked at the subscriber unit end by a resistance battery such as 'YB (Fig. 3) of subscriber unit SSUJ over resting contacts of the associated vertical hold relays ASH of the two subscriber units SSU9 and SSUIU forming the selected pair. The ASH relay'contacts are included in a series circuit to the marking battery in order to disconnect this batteryshould the subscriber units-be already in use either on an outgoing or an incoming call, but it'will be remembered that it was said that if the other subscriber unit of the pair was engaged on an outgoing call, then the in link need not be busi'ed and for this purpose the contacts ash3 of this other subscriber unit are short-circuited by OP AS11 at contacts 1083 (Fig. 3) 'disconnects the earth 7 Relay ICS in operating at contacts icsl extends earth over wiper and bank S054 to the switching relay 'SK in the common equipment SCEI and feed to all subscribers line relays to prevent the effective initiation of any other call onto asubscriber switching unit during this time, at contacts internal vertical bar circuit of either of the two ics l (Fig. 3) battery via resistance 'YC is connected to common lead ll extending to the lower incoming .test" winding of all line relays on the board in readiness for switching through to the j particular subscriber being called and. at 'icsS (Fig. 3) the earth circuit to the SA relays is opened. At contacts ics2 in the equipment FSC.

(Fig. 8) earth is extended forward over wiper and bank SCS3, contacts fz2 and wiper and bank DS'! in the common equipment SCEI, wiper DST occupying either of the positions H or 23, and then over-the Y lead to operate the incoming call horizontal bar relay 1? in the particular subscriber unit ssulo in which the called party is located, the Y lead for unit SSUI being shown. On operation of relay IP in unit SSUIIJ, the associated horizontal bar is actuated to prepare for the incoming connection. At the same time as earth is extended to the lower winding of relay SK an energising circuit is completed for the slow-to-operate relay G (Fig. 9) via contacts sk3 contacts if its internal link is involved on an out- 1 going call. H

Relay KR operates quickly and in so doing an operating circuit for relay KRR,(Fig. 9) over opens the circuit for magnet TM and completes the following circuit: battery, winding of relay KRR, contact krl, wiper andbank TSI, winding of relay N .to earth. Relay KRR in operating looks over its contacts lcrrl, further disconnects the driving'circuit for magnet TM at krrt and at krr2 extends earth over wiper TS2 in position it to bring up relay ICH which controls the first This relay in operating operates its associated vertical ban. whereupon since-horizontal bar relay ICP associated with the calling connecting link circuit AL will have already been operated, and since horizontal bar relay ETP, not shown, will also have been operated over wiper and bank DS3 in response lay SK will operate and will lock on its upper to the dialling of digit 0, then crossbar contacts will be operated to connect the connecting link. r circuit through the unit LSUI to the outgoing negative, positive and private conductors of the first in link. At contacts Ichl relay ICH prepares a holding circuit for itself, while at ich2 (Fig. 10) it marks .the internal vertical link as busy by disconnecting the circuit to KR over contacts of the banks of the switch TS pertinent to the first one of the four vertical links within the unit LSUI. Relay KR thereupon releases but relay KRR remains held and at contacts krrt and bank and wiper SCS3 to earth over, contacts ics2. During the operate time of relayG the selected line is tested over the following circuit:

. earth, icsi, S084, g4, and lower winding of relay S K- (Fig. 10), wiper'DS8 in position III or- 23,

wiper US'I in position and test lead T, such as that shown in connection with subscriber'No. 11, of the called line ill Assuming thatthe called line is free, a battery potential will be connected to the T lead via the I common resistance Y0 and contacts test (Fig. 5) f and then via the lower winding of the called subscribers' line relay which, in the present instance will be relay ISP on subscriber unit SSUIU. Re-

winding via contacts ski, ball and n6 to earth. At contacts slc2 a guard earth is extended forward over the T lead to operate the called subscriber's line relay iSP whereupon the associated. horizontal bar on unit SSUIU is operated so as further to prepare for connection-through to the called line. At contacts sk3 (Fig. 9) the energising circuit for the busy relay Gis opened so that this relay is prevented from operating and at sk l an energising circuit is completed for the slow-tooperate relay FZR. When relay FZR operates after a short interval, contacts fzri (Fig. 8) dis--,

' connect relay BA which commences to release slowly, contacts far! (Fig. 9) open the circuit of relays SGA and KRR, while contacts-fart (Fig.

(Fig. 8) extends earth .over common lead 23 and then via the series-connected contacts 30.3 of the SA relays in the two main 50-1ine subscriber groups 11- 50 and 61-00, winding of relay 'IGS to battery, the contacts sa3 serving to prevent relay 108 from operating should another subscriber be in process of being connected to a connecting link circuit at this stage and so serving to pr s vent the effective operation of the equipment FSC.

10) extend earth over wiper DSB in positions H or 23 and wiper TS! in position l8 and then over i a Q lead to operate the vertical bar relay ASH associated with the first of the four in? links available to the called unit -SSUIO, the in link and associated internal link inunit SSUIIJ having been previously selected by the TS switch when testing over lead X. Relay ASH in unit SSUIO in operating operates its associated vertical bar,

whereupon sets of contacts such as IP s? 1 i-g and i-s 1 as in Fig. 3 are-closed to-extend the calling connecting link circuit AL through the unit LSUI and over the "in" link to the required line. Earth via contacts M in the link circuit (Fig. 4) is now extended forward to operate the called subscriber's cut-off relay lK, which earth will also serve to 7 hold the vertical bar relay ICH in use in the unit LSUI. Relay IK in operating at its contacts lk3 extends a guardearth on to the incoming T lead and at contact lied and c2 completel disconnects the upper winding of the associated line relay from across the line in readiness. for the ringing ofthe called partys bell and of the subsequent answering by the called party.

During this time relay SGA (Fig. 9) will have released and at contacts sgal -(Fig. 8) earth is' extended upward over lead I! to bring up relay SD in the link circuit, while at sga! and sga3 (Fig.

- 8) the marking and starting potentials to the "common equipment FSC' are removed. Relay KRR in releasing at contacts krrl (Fig. 8) re- ,moves the start earth from common lead 23 exscribers on the board successfully to originatecalls. At contacts ics! (Fig. 8) and also at is! (Fig. 10) the earth extended forward over wipers and banks SCS3 and D51 over the Y lead to operate the incoming relay 1? in the called crossbar unit is removed and this relay releases and permits the associated horizontal bar to restore while the pin engaged by the vertical bar relay ASH remains held. Relay SD (Fig, 4) in-the link circuitin operating looks over its contacts sdl to the earthed P lead and at sd3 opens the energising circuit 'of. the link switching unit relay ICP, the associated horizontal bar of which, however, releases while the pin engaged by the vertical bar relay ICH remains held, while at sd 5 ringing tone is connected up over common lead 22-and over contacts b3, set and {2 to the centre winding of relay A from whence it extends by induction on to the incoming lines to the calling party. Relay SD in operating in the link circuit, at its contacts sdi and sd'l (Fig. 4), completes a circuit to ring the called party's bell as follows: earthed ringing current generator, common lead [5, winding of the ring trip relay F, contacts sdS and f5, and link switching -unit contacts on to the negative line, a return circuit being completed via the called party's bell and positive line via contacts'of the link switching unit and contacts f6 and sdl to battery via resistance YE, the calling subscriber receiving ringing tone during this tim over the circuit previously traced.

On release of relay BA in the equipment SCEI the starting and holding circuits for the time delay relays TA and T2 are opened and at contacts ball (Fig. 9) the holding circuit for relay SK is opened, while at the same contacts the earth circuit extended forward over wipers and banks DS6 and TS! over the Q lead to relay ASH is openedbut this relay now remains held over its own contacts I ash! (as in Fig. 3) to the incoming earthed P lead..

the home position I. The homing circuit will have already been completed for switch TS via its bank and wiper TSI (Fig. 9) on the release of relay KRR. Since the homing circuit of each of these switches is completed via the N relay, this nelay will remain held until all switches have homed and at its contact nl (Fig. 8) will disconnect the idle marking battery via resistance YH from lead It so as to mark the common equipment as busy. On release of relay N the marking battery is replaced on to common lead It and the equipment SCEI is now ready for further use by other connecting link circuits associated with unit LSUI.

When the called subscriber answers, relay F operates and at contacts i5 and f6 trips the rin ing andextends the lines through. At ,fl it completes a local locking circuit for itself to earth via contacts still and bl, while at 13 and it it connects the called subscriber's battery feeding relay D across the lines and at f2 it opens the ringing tone circuit.

Conversation now proceeds between the calling and called parties and at the end of conversation the connection will remain held until the calling party hangs up. When this occurs relay A releases and brings down relay B which in turn releases relays SD, F and D to free the link circuit,

and releases relay lClZ-I in the link switching unit,

while it also releases the called and calling subscribers cut-off relays and the vertical holding relays in the called and calling subscriber units, whereupon all equipment is free for further use.

Returning again to the testing of the called line, it will be remembered that this is efi'ected by relay SK in the common equipment SCEI during the slow-to-operate time of relay G. If the called line should prove to be busy, then relay'SK will not operate. and on the operation of relay G the operating circuit for relay SK is disconnected at contacts 94 (Fig. 10) and at contacts gl relay FZR is operated to release relays SGA, KRR and BA' to initiate release of the commonequipment in the manner already described. At contacts g3 (Fig. 10) the earth extended forward to operate the vertical hold relay in the called subscriber unit 'is disconnected and at 92 (Fig. 8) earth is extended upward over lead 18 to operate relay SE in the calling link circuit. Relay SE inoperating locks over its contacts sel, at se2 opens the holding circuit of relay ICP and at set extends busy tone over common lead l4 and contacts sdl and f2 to the centre winding of relay A to return busy tone to the calling party, .The link circuit remains, held until the calling party hangs up when relays terval during which time a second common equipment is taken into use by another calling party, then if the dialling operations of the two subscribers should overlap it will not be possible in the case where the two called parties involved are on the same subscriber unit for the two SCE equipments concerned to operate on the one crossbar subscriber unit, since the equipment FSC which controls the testing of 9. called line from an SCE equipment can only be associated with each of said primary lines is selectively connect-' able-to any of said links and each of said secondary lines is connectable only to its corresponding link, apparatus common to all of said groups, means for causing said apparatus to search in any particular one of said groups for an idle secondary line whose corresponding link also is idle,

means in said apparatus efiective if such a line is found to operate said crossbar switch to con- .dition all of the secondary lines in said particular group for inclusion-in a connection, means for alsooperating said crossbar switch to condition anyone of said primary lines for inclusion in a connection, said -switch. having an individualmagnet 'for each of said links, andlmeans efiec- *"tive'while said primary line andsaid secondary ble to any of said links and each. of said secondary lines is connectable only to its corresponding link, said switch having a different selecting magnet for each of said primary lines, means for operating any one of said magnets thereby to condition the primary line corresponding thereto be included in a connection, a magnet common to said secondary lines at times operated while said primary line is thus conditioned, thereby also to condition all of said secondary lines to be included in a connection, said crossbar switch having a difierent holding magnet for each .of said links, and means for operating any one of said holding magnets while said primary line and said secondary lines are concurrently conditioned, said crossbar switch thereupon efiective to complete a connection only from said conditioned primary line, over the link corresponding to the operated holding magnet, to the secondary line corresponding to that link. v r a 4. In a switching system, a plurality of primary lines, a plurality of groups oi'isecondary lines,

lines are concurrently conditioned to operate the "magnet for" the link corresponding to said idle secondary line; said --crossbar switch thereupon *"effective to complete a connection from saidconditioned "primary line tosaididle secondary line over the "link:- corresponding" to said idle second- -arylinewithout completing any other connection between any primary-line and any secondary line.

2. In a switching system,- a pluralityof primary lines, a plurality of links; a plurality of groups of secondary lines, the different secondary lines of j each group corresponding respectively to the dif- 'ferent ones of said links, a single crossbar switch "in which all of said lines and all of said links appear, the, contactsof said'switch being so arranged that each of said primary lines is selectively connectable to any of said links and each of said secondary lines isc'o'nnectable only to its corresponding link, said switch having a diiferent selecting magnet for each of saidprimary lines,

means for operating any one of'said magnets thereby to condition the primary line corresponding thereto to be included in a connection, said switch also having a diiferent selecting magnet for each said group of secondary lines, means ioroperating any one of said last magnets while secondary lines are concurrently conditioned, said crossbar switch thereupon efiective to complete a con'nection'only from said conditioned primary line, over thelink corresponding to the operated holdifig magnet, to the conditioned secondary line' a corresponding to that link.

3, In a switching system, a plurality or primary lines, a plurality of secondary lines, a plurality offlinks, the different ones of said links corresponding respectively to the different ones of said secondary lines, a single crossbar switch in which all of said lines and all of said links appear, the

each of said primary lines-is selectively connectas the different secondary lines of each group corresponding respectively to the different ones of said primary lines, a crossbar switch in which all of said lines appear, the contacts or said switch being so arranged that each of said secondary lines is connectable only to its corresponding primary line, app ratus common to all of said groups, means for causing said apparatus to searchiri. any particular one oi' -said groups for an idle secondary line whose corresponding primary line also is idle, meanscontrolled by said apparatus if such a line is found 'for operating said switch to condition all of the secondary lines in said particular group to be connected over certain of said contacts to their respective primary lineswithout thus conditioning the secondary lines 'ofany other group, a different switch-operating magnet for each primary line, means controlled by said apparatus .while the secondary lines of said particular group are thus conditioned for operating the magnet for theprimary line corresponding to the idle secondary line which was found, said crossbar switch con- 5. In a switching systenna plurality of primary lines, a plurality of groups or secondary lines,

- the different secondary lines 'of each group correholding magnets while said primary line and said sponding respectively to the difierent ones of' said primary lines, a set or contacts individual to each secondary line for connecting that secondary line to its corresponding primary line, I

means operated for conditioning the contact sets individual to all of the secondary lines of any particular group to complete connections from those secondary lines to their respectively corresponding primary lines without thus conditioning conditioned, and means controlled by each magcontacts ofsaid switch being so arranged that v6. In a switching system, a plurality of primary 

